1. Field of the Invention
This invention relates to a lithium ion-conductive solid electrolyte, a process for the production thereof, a sheet of the solid electrolyte, and a structure in which the solid electrolyte sheet is used. More particularly, this invention relates to a novel lithium ion-conductive solid electrolyte having a high lithium ion-conductivity at room temperature and suitable for use in solid electrochemical devices such as solid microcell and the like, a process for the production thereof, a sheet of the solid electrolyte, and a structure in which the solid electrolyte sheet is used.
2. Discussion of the Background
The technical progress in electronic industry in recent years is remarkable and electronic components such as IC, LSI and the like are much used in almost all fields. Such technical progress is also seen in the field of cell technique and an attempt has been made to render cells smaller and thinner. Such cells are used in large amounts as a power source for card-shaped portable calculator, camera, wrist watch, etc.
Most of the cells used in the above applications are alkali cells or lithium cells, and a liquid electrolyte is used therein. These cells in which a liquid electrolyte is used require highly efficient sealing and the current sealing technique mainly employed is a crimp seal with a gasket. In this sealing technique, however, the thinner the cells, the larger the proportion of the sealing material occupying the cell volume, which makes it difficult to provide the required cell capacity. Therefore, there is a limitation in making cells thinner.
Under such circumstances, research is being conducted on solid electrolytes having a high lithium ion-conductivity in order to make cells smaller and thinner.
As such a solid electrolyte, polymer solid electrolytes have been tested and a typical example thereof is a poly(oligooxyethylene methacrylate)-alkali metal salt system. However, the ion-conductivity thereof is at best about 10.sup.5 S/cm at room temperature and moreover this system is inferior in selectivity of movable ions and causes the movement of not only cations (e.g. Li.sup.+) but also anions (e.g. ClO.sub.4.sup.-). Therefore, polymer solid electrolytes have not been used in practice.
Vigorous research is also being conducted on powders of inorganic solid electrolytes such as lithium halides; mixtures of a lithium halide with aluminum oxide; lithium nitride; lithium nitride derivatives; oxyacid salts of lithium having a .gamma..sub.II -Li.sub.3 PO.sub.4 -type structure; and the like. However, lithium halides and mixtures of a lithium halide with aluminum oxide have inferior chemical stability and a low conductivity .sigma. of 10.sup.-6 -10.sup.-7 S/cm. Lithium nitride and its derivatives have a high conductivity .sigma. of about 10.sup.-3 S/cm but poor chemical stability. The oxyacid salts of lithium having a .gamma..sub.II -Li.sub.3 PO.sub.4 type structure have excellent chemical stability but a low conductivity .sigma. of about 10.sup.-5 S/cm. Therefore, the application of these inorganic solid electrolytes to various fields is difficult.
Furthermore, the inorganic solid electrolytes are powdery, and hence, require pelletization at high pressures when they are formed into cells or the like. This is a great obstacle in achieving high productivity, uniformity, etc. Moreover, the pellets obtained are hard and brittle, and therefore, there is a limit to cell thinness and it is difficult to produce a cell having a large surface area. Also, in the application of the inorganic solid electrolyte to cells, it is necessary to apply a great pressure to intimately contact the solid electrolyte with an electrode active substance when adhering the two to each other. Therefore, workability, adhesion, etc. are varied. When the adhesion between the inorganic solid electrolyte and electrode active substance is made in a large area, no uniform adhesion is obtained and the electrolyte pellets are broken.